Moles grow hair because they often form over or near hair follicles, which are the tiny structures in the skin responsible for producing hair. The hair follicles within a mole can remain active or become more active, leading to the growth of thicker, darker hair from the mole. This happens due to several factors including hormonal influences (such as during puberty), increased blood supply to the mole, and genetic factors. The presence of hair in a mole generally indicates that the mole is benign and healthy, as cancerous moles tend to disrupt normal hair follicle function. However, hairy moles should still be monitored for any changes in size, shape, or color that might indicate a problem.

In summary, moles grow hair because they contain active hair follicles that continue to function normally or become more stimulated within the mole, allowing hair to grow from the mole itself.